Sea Cucumber-like Polyaniline Nanofibers Synthesized by Aqueous Solution Method

Transcription

1 J. Mater. Sci. Technol., 2010, 26(1), Sea Cucumber-like Polyaniline Nanofibers Synthesized by Aqueous Solution Method Baoyun Liu 1,2), Li Liu 1), Nanlin Shi 1), Jun Gong 1) and Chao Sun 1) 1) State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang , China 2) Graduate School of Chinese Academy of Sciences, Beijing , China [Manuscript received March 11, 2009, in revised form July 9, 2009] The preparation of polyaniline nanofibers in aqueous solution was studied as functions of the concentrations and ratios of reactants. The morphology and microstructure of polyaniline nanofibers are affected by the concentrations and proportions of the reactants. A special kind of sea cucumber-like polyaniline nanofibers can be prepared under control of reaction conditions. Secondary growth is the formation mechanism. In addition, the bulk electrical conductivity of these sea cucumber-like polyaniline nanofibers was higher than that of other common polyaniline nanofibers. KEY WORDS: Polyaniline; Nanofiber; Aqueous solution method; Morphology; Electrical conductivity 1. Introduction Polyaniline nanofibers have attracted much attention because of their potential applications such as chemical and biological sensors [1,2], surface modified electrode [3], biocompatible material [4], artificial muscles [5], and anticorrosion coating [6]. Many synthesis methods [7 12] have been reported to prepare polyaniline nanofibers. The morphology of polyaniline nanofibers can be modified by introducing hard templates [13,14] or structure directing molecules [15 17]. Among these methods, the aqueous solution method is still the preferred way to prepare polyaniline nanofibers for its advantages of easy synthesis, manageable processing and easy industrialization. Recently, considerable efforts have been made on synthesis of polyaniline nanofibers with more complicated structures. Pang et al. [18] reported that polyanilinevanadium oxide nanocomposite achieved tremella-like nanosheets. In the subsequent work, Li et al. [19] syn- Corresponding author. Prof., Ph.D.; Tel.: ; Fax: ; address: csun@imr.ac.cn (C. Sun). thesized polyaniline nanofibers with different surfaces by using vanadic acid as oxidant in aqueous solution. They considered that the lower oxidant/reduction potential of vanadic acid forms nanorods on the nanofibers surfaces. Du et al. [20] found that branched polyaniline nanofibers could be prepared by solidstate mechanochemical route. When the reaction system transferred into solution, polyaniline nanofibers with different surfaces by using ferric chloride could be obtained [21]. These polyaniline nanofibers with rough or smooth surface could be synthesized by altering reaction temperature or time. Both Li [19] and Du [20] considered that the specific oxidants play an important role in synthesizing this specific morphology of polyaniline, and the reaction parameters are not discussed except temperature and time. In our recent study, a kind of polyaniline nanofibers with nanospikes all over the surfaces, named as sea cucumber-like polyaniline nanofibers were obtained. Their morphology is similar to those synthesized with vanadic acid and ferric chloride oxidants. These sea cucumber-like polyaniline nanofibers are synthesized in aqueous solution with both toluene-

2 40 B.Y. Liu et al.: J. Mater. Sci. Technol., 2010, 26(1), sulfonic acid monohydrate and hydrochloric acid and common oxidant ammonium peroxydisulphate. Any porous templates or molecular directing agents are not used during the polymerization. The influence factors on the morphology of polyaniline nanofibers were investigated including aniline concentration, the ratio of aniline and acid and the species of acid. Details of our work were reported in this paper and the growth mechanism was also discussed. 2. Experimental Aniline (An) was used as monomer and purified before reaction. Ammonium peroxydisulphate (APS) was used as oxidant. Toluene-4-sulfonic acid monohydrate (TSA) and hydrochloric acid (HCl) were used as dopants. Acetone and distilled water were employed in washing treatment. All chemicals were of analytical reagent grade. In a typical experiment, aniline and acid were dissolved in 20 ml distilled water. The mixture was continuously stirred and cooled to 0 5 C in ice-water bath. APS water solution was dropped into the previous mixture. The reaction compounds were stirred sequentially for 24 h to finish the polymerization after the addition of APS. The reaction mixture was filtrated and washed with distilled water and acetone separately until the filtrate became colorless. The solid cake was dried for 48 h at 50 C, and then ground into powder and stored at room temperature. The micrographs of polyaniline were obtained by scanning electron microscopy (SEM) (Philips XL- 30FEG and Nova NanoSEM 430 and LEO SUPRA 35) and transmission electron microscopy (TEM) (JEOL JEM-2010 HR). The samples were ultrasonic dispersed in ethanol before the examinations. The bulk electrical conductivity (σ) of polyaniline was measured with the linear four-point probe method at room temperature. The sample was ground and pressed into the disc under the pressure of 8 MPa. Voltages (V ) were obtained under a preset electric current (I) and the values of σ were calculated with the following equations [22] : σ = 1 ρ (1) ρ = C V I F 1F 2 (2) where ρ is the electrical resistivity, C the coefficient of probe, F 1 the correction factor of thickness and F 2 the correction factor of diameter. The X-ray diffraction patterns of polyaniline were taken on a diffraction spectrometer (Rigaku D/MAX-2500 PC) with monochromatic CuKα (wavelength= nm) radiation. The Raman spectra were taken from a Raman spectrometer (Jobin Yvon HR800) at nm. 3. Results and Discussion Figure 1 shows SEM and TEM images of polyaniline nanofibers prepared using APS as oxidant with Fig. 1 SEM and TEM images of the sea cucumber-like polyaniline nanofibers doped with TSA prepared with 0.3 M aniline concentration, TSA/An=1.6:1 and APS/An=1:1: (a) SEM image in lower magnification, (b) SEM image in higher magnification, (c) TEM image 0.30 mol/l aniline and a ratio of TSA/aniline of 1.6 at 0 5 C. A large quantity of polyaniline nanofibers with diameters of nm and length of 1 2 µm were observed (Fig. 1(a)). In higher magnification SEM image (Fig. 1(b)), we found these nanofibers are covered with nanospikes all over the surfaces and named them as sea cucumber-like polyaniline nanofibers. The TEM image (Fig. 1(c)) also confirmed the presence of nanospikes. The influences of experiment parameters on the morphology of polyaniline have been investigated. Figure 2 shows SEM images of polyaniline nanoparticles synthesized with various aniline concentrations. Most of the products are nanospheres and nanorods with 0.15 mol/l aniline (Fig. 2(a)). When aniline concentration is increased to 0.20 mol/l, polyaniline nanofibers with smooth surfaces can be synthesized

3 B.Y. Liu et al.: J. Mater. Sci. Technol., 2010, 26(1), Fig. 2 SEM images of polyaniline nanofibers doped with TSA prepared with various aniline concentrations, TSA/An=1.6:1 and APS/An=1:1: (a) 0.15 mol/l aniline, (b) 0.2 mol/l aniline, (c) 0.35 mol/l aniline, (d) 0.4 mol/l aniline.) (Fig. 2(b)). The sea cucumber-like polyaniline nanofibers is synthesized with 0.30 mol/l aniline concentration (Fig. 1). However, when aniline concentration is continuously increased, the products turn back to nanofibers with smooth surfaces with 0.35 mol/l aniline (Fig. 2(c)) and nanorods with 0.40 mol/l aniline (Fig. 2(d)). Moreover, it was noticed that polyaniline nanofibers with smooth surfaces were 100 nm in diameter and 1 µm in length respectively, which were thinner and shorter than those of the sea cucumber-like polyaniline nanofibers. These results indicate that aniline concentration plays an important role on the morphology and diameters of polyaniline nanoparticles. The nanospikes are only formed at a certain narrow range of aniline concentration, but disappear by either raising or declining it. In order to investigate the influence of doping acid, we modified the molar ratio of TSA to aniline. Figure 3 shows that the products are the sea cucumber-like polyaniline nanofibers when the ratio of TSA to aniline is decreased from 1.6 to 0.8 with 0.20 mol/l aniline (Fig. 3(a)) or increased to 1.8 with 0.35 mol/l aniline (Fig. 3(b)) respectively. Only polyaniline nanofibers with smooth surfaces were prepared without changing the ratio of TSA to aniline with these two aniline concentrations. Moreover, it was found that the sea cucumber-like polyaniline nanofibers can also be obtained by using HCl besides organic acid TSA (Fig. 4). These above results indicate that the forma- Fig. 3 SEM images of the sea cucumber-like polyaniline nanofibers doped with TSA prepared with various molar ratios of TSA to aniline at different aniline concentrations: (a) TSA/An=0.8:1 and 0.2 mol/l aniline, (b) TSA/An=1.8:1 and 0.35 mol/l aniline

4 42 B.Y. Liu et al.: J. Mater. Sci. Technol., 2010, 26(1), Table 1 Bulk electrical conductivity of PANi-TSA with different morphology and reaction condition. (C: sea cucumber-like polyaniline nanofibers, S: polyaniline nanofibers with smooth surfaces, both of TSA/An and APS/An are molar ratios.) Aniline/(mol/L) TSA:An APS:An σ/(s/cm) morphology :1 1: C :1 1: S :1 1: C :1 1: S :1 0.6: C Fig. 4 SEM image of the sea cucumber-like polyaniline nanofibers doped with HCl (0.35 mol/l aniline, HCl/An=1.6:1 and APS/An=1:0.8.) tion of the sea cucumber-like polyaniline nanofibers also depends on the amount of doping acid. There is a certain value of TSA/aniline to produce the specific nanostructure as aniline concentration. However, the species of acid has no influence on the formation of the sea cucumber-like polyaniline nanofibers. They can be prepared by both of common organic and inorganic acid. The structure of doping acid has no obvious effect. In summary, the preparation of the sea cucumber-like polyaniline nanofibers depends on the optimized selection of concentration and proportion of the reactants. They can be synthesized by common oxidant and doping acid in appropriate reaction condition. The bulk electrical conductivities of polyaniline nanofibers at room temperature are determined by standard four-point probe method. As shown in Table 1, the bulk conductivity of the sea cucumber-like nanofibers is 3 5 times higher than that of polyaniline nanofibers with smooth surfaces. We consider that the promotion in bulk electrical conductivity of the sea cucumber-like polyaniline nanofibers results from their specific morphology. The nanospikes can increase the contact surfaces of different nanofibers after pressing into disc. In addition, Fig. 5 shows the presence of nanowires of approximately 20 nm in diameter among the sea cucumber-like nanofibers. These nanowires connect different nanofibers and act as bridges of electrical current. The electrical resistance of the contacts [23] between nanofibers is decreased by these bridges. Thus the bulk electrical conductivity Fig. 5 SEM images of the sea cucumber-like polyaniline nanofibers doped with TSA is increased. The partially crystallized structure [24,25] of the sea cucumber-like polyaniline nanofibers synthesized in this study was observed. The high resolution TEM (HRTEM) image is shown in Fig. 6. It is clearly shown that the crystallized regions distributes in the amorphous region. Figure 7 shows XRD patterns of the sea cucumber-like polyaniline nanofibers and polyaniline nanofibers with smooth surfaces. Three typical broad peaks at 9, 15, 20 and one intense at 25 [25] are observed. It confirms the coexistence of crystallized and amorphous state and consists with the result of HRTEM. Comparing to polyaniline nanofibers with smooth surfaces, the sea cucumber-like polyaniline nanofibers have a shoulder at 27 and relatively stronger peaks at 15 and 25. It indicates that the sea cucumber-like polyaniline nanofibers has better crystallinity than polyaniline nanofibers with smooth surfaces. The Raman spectra of polyaniline nanofibers at nm are shown in Fig. 8. The bands of the sea

5 B.Y. Liu et al.: J. Mater. Sci. Technol., 2010, 26(1), =632.8 nm Intensity / a.u a b Wavenumber / cm -1 Fig. 6 High-resolution TEM image of the sea cucumberlike polyaniline nanofibers doped with TSA Fig. 8 Raman spectra of polyaniline nanofibers doped with TSA at nm, a: sea cucumber-like polyaniline nanofibers, b: polyaniline nanofibers with smooth surfaces 25 Intensity / a.u a b Fig. 9 Sketch for the formation mechanism of the sea cucumber-like polyaniline nanofibers / deg. Fig. 7 XRD patterns of polyaniline nanofibers doped with TSA, a: sea cucumber-like polyaniline nanofibers, b: polyaniline nanofibers with smooth surfaces cucumber-like polyaniline nanofibers shifted to higher wavenumber compared to the spectra of polyaniline nanofibers with smooth surfaces. The relative intensity of the bands at 582, 1355, 1385, 1598, 1645 cm 1 become stronger. It was reported [26] that the band at 1355 cm 1 was associated to νc N of polaron and the bands at 582, 1385, 1645 cm 1 related to the formation of the cross-linking structures. The promotion indicates that the doping degree and crosslinking of the cucumber-like nanofibers are both increased. The microstructure of polyaniline changes with the variation of the morphology. The enhanced crystallinity, doping degree and cross-linking degree of the sea cucumber-like polyaniline nanofibers result in the higher electrical conductivity. As we know, the formation of polyaniline nanofibers follows the classical theory of nucleation and growth [27,28]. When aniline and acid were dissolved in water, they can form micelles [29]. At the same time, parts of aniline still exist as free monomers. The concentration of aniline and the ratio of acid determine the amount and ratio of micelles and free monomers. When aniline concentration is low (below 0.15 mol/l in this study), both of the amount of micelles and monomers are small. Polyaniline only grows to nanospheres or nanorods (Fig. 2(a)). When aniline concentration is increased, there are enough free monomers to make polyaniline grow along the axes of molecular chain and form nanofibers (Fig. 2(b)). When aniline concentration is increased continuously, the growth along the vertical axes of the molecular chain cannot be restrained anymore because of large numbers of micelles and free monomers. The secondary growth [30] occurs and results in wider and longer nanofibers and also the nanospikes on their surfaces (Fig. 1(b)). Some nanospikes grow longer and form the nanowires between the nanofibers. Figure 9 is the sketch for the formation of the sea cucumber-like polyaniline nanofibers. However, the solubility of aniline in water is a definite value under a preset temperature (3.4%, 20 C). The number of free monomers cannot be increased anymore when aniline concentration is increased over a certain level. The increase in the ratio of micelles to monomers results in turning back the morphology of polyaniline. The ratio of micelles to monomers can

6 44 B.Y. Liu et al.: J. Mater. Sci. Technol., 2010, 26(1), also be changed by adjusting the ratio of acid to aniline. when the ratio of acid to aniline at lower aniline concentration is decreased or the ratio of acid to aniline at higher aniline concentration is increased, sea cucumber-like polyaniline nanofibers are obtained (Fig. 3). It proves the ratio of micelles to monomers also plays an important role in the formation of the sea cucumber-like polyaniline nanofibers. In other words, the formation of the sea cucumber-like polyaniline nanofibers is only related to the concentration and the ratio of aniline to doping acid and not the species of acid and oxidant. 4. Conclusion A kind of sea cucumber-like polyaniline nanofibers was prepared by aqueous solution method. It can be synthesized by common and cheap oxidant APS and doped with both organic and inorganic acid. Their formation is sensitive to the variation of the concentration and the ratio of aniline to doping acid. The species of acid and oxidant do not have obvious influence on it. Secondary growth is the essential reason for the formation of the sea cucumber-like polyaniline nanofibers. The results of TEM, XRD and Raman spectra indicate that crystallinity, doping degree and cross-linking degree of the sea cucumber-like polyaniline nanofibers are higher than those of polyaniline nanofibers with smooth surfaces. Moreover, the bulk electrical conductivity of the sea cucumber-like polyaniline nanofibers is much enhanced. Acknowledgement This work is supported by the National Advanced Materials Committee of China (No. 2008AA03Z409). REFERENCES [1 ] J.X. Huang, S. Virji, B.H. Weiller and R.B. Kaner: J. Am. Chem. Soc., 2003, 125, 314. [2 ] H. X. Chang, Y. Yuan, N.L. Shi and Y.F. Guan: Anal. Chem., 2007, 79, [3 ] J.H. Park and O.O. Park: J. Power Sources, 2002, 111, 185. [4 ] N.K. Guimard, N. Gomez and C.E. Schmidt: Prog. Polym. Sci., 2007, 32, 876. [5 ] Y.A. Ismail, S.R. Shin, K.M. Shin, S.G. Yoon, K. Shon, S.I. Kim and S.J. Kim: Sensors Actuators B- Chemical, 2008, 129, 834. [6 ] J. Fang, K. Xu, L.H. Zhu, Z.X. Zhou and H.Q. Tang: Corros. Sci., 2007, 49, [7 ] F. Yakuphanoglu, E. Basaran, B. F. Senkal and E. Sezer: J. Phys. Chem. B, 2006, 110, [8 ] J. Wang, S. Chan, R.R. Carlson, Y. Luo, G.L. Ge, R.S. Ries, J.R. Heath and H.R. Tseng: Nano Lett., 2004, 4, [9 ] S.H. Lee, D.H. Lee, K. Lee and C.W. Lee: Adv. Funct. Mater., 2005, 15, [10] X.Y. Zhang, R. C.Y. King, A. Jose and S.K. Manohar: Synth. Met., 2004, 145, 23. [11] X.Y. Zhang, W.J. Goux and S.K. Manohar: J. Am. Chem. Soc., 2004, 126, [12] A.G. Macdiarmid and A.J. Epstein: Synth. Met., 1994, 65, 103. [13] H.J. Ding, X.M. Liu, M.X. Wan and S.Y. Fu: J. Phys. Chem. B, 2008, 112, [14] Y. Gao, X. Li, J. Gong, B. Fan, Z.M. Su and L.Y. Qu: J. Phys. Chem. C, 2008, 112, [15] Y. Yan, Z. Yu, Y.W. Huang, W.X. Yuan and Z.X. Wei: Adv. Mater., 2007, 19, [16] W.G. Li and H.L. Wang: J. Am. Chem. Soc., 2004, 126, [17] C.Q. Zhou, J. Han, G.P. Song and R. Guo: Macromolecules, 2007, 40, [18] S.P. Pang, G.C. Li and Z.K. Zhang: Macromol. Rapid Commun., 2005, 26, [19] G.C. Li, L. Jiang and H.R. Peng: Macromolecules, 2007, 40, [20] X.S. Du, C.F. Zhou, G.T. Wang and Y.W. Mai: Chem. Mater., 2008, 20, [21] X.S. Du, C.F. Zhou and Y.W. Mai: J. Phys. Chem. C, 2008, 112, [22] Electronic Field Standard of PR China, SJ/T , [23] Y. Long, Z.J. Chen, N.L. Wang, Y.J. Ma, Z. Zhang, L.J. Zhang and M.X. Wan: Appl. Phys. Lett., 2003, 83, [24] L. Mazerolles, S. Folch and P. Colomban: Macromolecules, 1999, 32, [25] J.P. Pouget, M.E. Jozefowicz, A.J. Epstein, X. Tang and A.G. Macdiarmid: Macromolecules, 1991, 24, 779. [26] G.M. do Nascimento and M.L.A. Temperini: J. Raman Spectrosc., 2008, 39, 772. [27] N.R. Chiou and A.J. Epstein: Adv. Mater., 2005, 17, [28] B.C. Bunker, P.C. Rieke, B.J. Tarasevich, A.A. Campbell, G.E. Fryxell, G.L. Graff, L. Song, J. Liu, J.W. Virden and G.L. Mcvay: Science, 1994, 264, 48. [29] Z.M. Zhang, Z.X. Wei and M.X. Wan: Macromolecules, 2002, 35, [30] H. Liu, X.B. Hu, J.Y. Wang and R.I. Boughton: Macromolecules, 2002, 35, 9414.